Using the same formula, calculate amp draw for 3-phase WYE with 3 transformers, each connected to a 7200 volt Hot and Neutral.
The primary volts E1 is the same 7200 volts.
Let's say the main breaker is same 250 amps.
But let's change the secondary voltage E2 to 480 volts commonly found in a commercial 3-phase service. The turns ratio is 15:1 instead of 30:1 because 7200 divided by 480 equal 15.
This means primary amperage I1 is 250 amp divided by 15 = 16 amps. This means the 7200 volt power line delivers 16 amps to the commercial building.
Except a 3-phase service has 3 transformers instead of a single household transformer ... and each of the 3 transformers pulls power from a different Hot wire on the 3-phase distribution ... so the commercial building pulls 16 amps from 3 separate Hot wires ... receiving 3 x 16 amps (3 Hot legs) for each rotation of the generator, while residential single phase receives 2 x 8 amps (2 Hot legs at 240 volt)  for each rotation of the generator. Meaning that 480 volt 3-phase delivers more power to the commercial building than using single-phase. Plus, average voltage for 3-phase never has two wires cross zero at same time, which yeilds more power and higher efficiency, making it easier for the generator to supply high voltage 3-phase service.